The present invention relates, in general, to hydraulic reservoirs for vehicle lift systems, and in particular, to "full hydraulic" air-oil tanks with internal low oil control mechanisms on vehicle lift systems.
Hydraulic lift systems for vehicles are well-known. In addition to well-known "semi-hydraulic" or so-called "airdraulic" lift systems, in which air is forced into and over an oil-filled piston within a lift cylinder, "full-hydraulic" lift systems are also well-known designs for such vehicle lifts. In a full hydraulic lift system, only hydraulic oil is present within the lift cylinder and forces the lift piston to rise from the lift cylinder as pressurized oil is introduced into the cylinder.
A full hydraulic lift system requires, as a component part, an air-oil reservoir tank coupled by pipe to the lift cylinder. As air is introduced into the reservoir tank, oil is forced therefrom and into the lift cylinder, raising the lift piston. While there is no requirement as such, in many full hydraulic lift systems the air-oil reservoir tank is buried underground beside the lift cylinder to conserve work space above ground.
It is also well-known that since February, 1974, ANSI standard B153.1 requires some form of low oil control mechanism as a part of a full hydraulic lift system in order to prevent the entry of air into the lift cylinder for safety reasons. If the oil level within the reservoir tank, which necessarily falls and rises as the lift piston raises and lowers as oil from the reservoir tank is forced into the lift cylinder, becomes too low, the air that is being forced into the reservoir tank will enter the lift cylinder. Because air is a compressible fluid, unlike hydraulic oil, a hydraulic lift under load with air in the lift cylinder, as with a vehicle atop the lift, will tend to rapidly propel the lift piston from the lift cylinder when the load is removed, i.e., driven off the lift, due to the rapid expansion of the air in the lift cylinder with the removal of the load. To avoid this unsafe condition, full hydraulic lift systems provide a low oil control mechanism to shut off the flow of oil from the reservoir tank to the lift cylinder when the oil level in the reservoir tank is below a predetermined level, thereby preventing air from entering the lift cylinder.
One well-known low oil control mechanism employs a perforated guide tube extending from the top of the reservoir tank, with an opening therein for filling the tank with oil and for maintenance, to the bottom of the tank. A hollow float, having a positive buoyancy in the oil, rides within the guide tube for resting upon and blocking a valve seat at the bottom of the tank. It shall be understood that the phrase "having a positive buoyancy in the oil," as used herein, refers to the tendency of the float to rise to the surface of the oil when submerged therein. A connecting pipe, extending through the bottom of the tank and joining to the guide tube, with the valve seat located therein at the junction with the guide tube, joins the reservoir tank with the lift cylinder and has various control valves interposed therein for regulating and controlling the flow of oil to and from the reservoir tank and lift cylinder.
Such a design, with the connection between the lift cylinder and the reservoir tank joining the reservoir tank at the bottom thereof, has several problems. First and most important, in some localities the presence of moisture, electrolytes, and stray currents within the soil causes corrosion of the connecting pipe attached to a buried reservoir tank. The corrosion effects increase with distance from the surface of the ground, due to the presence of increased moisture and electrolytes at greater depths, and are rarely seen near the ground surface. The corrosion damage is therefore most acute at the bottom of the reservoir tank where the connecting pipe joins with reservoir tank. Known solutions to this corrosion problem include the use of a cathodic protection system or the wrapping of the connection pipe in a protective coating of tape. In practice, though, such protection is difficult to achieve, as the connecting pipe is not joined to the reservoir tank until after the reservoir tank is deep within an excavated hole. Working in close quarters down in the excavated hole, workmen occasionally inadvertently leave unprotected areas or "holidays" on the connecting pipe as they attempt to wrap the pipe in protective tape. Such unprotected areas act to concentrate the corrosion in localized exposed regions on the pipe, causing, in fact, more rapid failure than would be otherwise seen with a completely bare connecting pipe. Even though test equipment, such as dielectric strength meters, exist for testing the protection of a pipe, the use of such equipment is cumbersome down in a hole on a construction site.
Another problem, alluded to above, is the difficulty of fastening a connecting pipe to the bottom fitting on a reservoir tank. And, since the connecting pipe must pass through control valves at the surface of the ground, various elbows and fittings must be installed therein to allow the connecting pipe to bend from the bottom of the reservoir tank and rise to the ground surface. Because the reservoir tank may only be secured to the connecting pipe after placement within an excavated hole, a worker performing the installation often must join the connecting pipe and various fittings in a confined space, frequently producing suboptimal results such as a leaky joint at one of the multiple connecting pipe elbows or fittings.
Finally, the necessity of providing a connection to the tank at a bottom fitting prevents the manufacturer of the tank from completely encasing or sealing the lower end of the tank with protective material, such as fiberglass reinforced plastic, during manufacture in order to protect the tank from corrosion. Because protection can only be applied once the reservoir tank is lowered into its excavation hole and the connecting pipe is attached, protection of the tank during manufacture is precluded.
It is therefore desirable to have a air-oil full hydraulic reservoir tank including internal low oil control means, that minimizes the above-mentioned problems.